Electronic amplifiers, and in particular power amplifiers, are widely used in audio equipment and in the transmitters of telecommunication transceivers. One of the requirements for such amplifiers is to obtain the best possible efficiency. For the purpose of this description, amplifier efficiency can be regarded as the ratio between alternating-current (AC) output power and direct-current (DC) input power supplied to the amplifier.
In known amplifiers, this requirement is addressed by first specifying the required output power level, and then optimising the amplifier for that specific output power level. One known way of optimising the amplifier is to select an optimum load for the required output power level. Another is to employ a switchable load that can be adjusted according to different required output power levels. Usually, the output power level, in the case of a single value of load, is specified to be either the highest operating level, or slightly below the highest operating level in order to minimise the effect of reduced efficiency, measured as lost dissipated power, at higher power levels for which dissipated loss is greatest.
However, a problem of this approach is that it is only possible, for a given load, to maintain near-optimum efficiency for the specified output power level for that load. If an amplifier optimised for a specified output power level is used for a different power level, then a significant reduction in efficiency results if the amplifier is not re-adjusted (e.g. by adjusting the load). For example, if a class-A amplifier with a fixed supply voltage is optimised for an output power level of P Watts, then the amplifier, when operated at that output power level P, will have an efficiency that approaches the ideal 50% (the theoretical maximum for a transistor saturation voltage Vsat of zero). If the same class-A amplifier is operated at an output power level of P/2 Watts, then the theoretical best obtainable efficiency is 25%.
The invention aims to provide an amplifier whose efficiency can be optimised for a range of output power levels.